BJT structure

note: this is a current of electrons (npn case) and so theconventional current flows from collector to emitter.

heavily doped ~ 10^15provides the carriers

lightly doped ~ 10^8 lightly doped ~ 10^6

BJT characteristics

BJT characteristics

BJT modes of operation

Mode EBJ CBJ

Cutoff Reverse Reverse

Forward active Forward Reverse

Reverse active Reverse Forward

Saturation Forward Forward

Cutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch.

Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βf in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.

Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the βf in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

BJT modes of operation

BJT structure (active)

current of electrons for npn transistor –

conventional current flows from collector to emitter.

BB

CCEE

IIEE IICC

IIBB

--

++

VVBEBE VVCBCB--

++

++-- VVCECE

BJT equations (active)

T

BEV

V

SC eIi

T

BEV

VS

E eI

i

E

C

i

i

= Common-base current gain (0.9-0.999; typical 0.99)= Common-base current gain (0.9-0.999; typical 0.99)

BJT equations (active)

T

BEV

V

SC eIi

T

BEV

VS

B eI

i

B

C

i

i

= Common-emitter current gain (10-1000; typical 50-200)= Common-emitter current gain (10-1000; typical 50-200)

BJT equations (active)

11

= Common-emitter current gain (10-1000; typical 50-200)= Common-emitter current gain (10-1000; typical 50-200)

= Common-base current gain (0.9-0.999; typical 0.99)= Common-base current gain (0.9-0.999; typical 0.99)

BJT large signal models (forward active)

forwardF ~

BJT large signal models (reverse)

reverseR ~

Common-base current gain (0.1-0.5) BJT transistor is not a symmetrical deviceCommon-base current gain (0.1-0.5) BJT transistor is not a symmetrical device

BJT Ebers-Moll (EM) model

BJT structure

The npn transistor has beta=100 and exhibits an Ic=1mA at VBE=0.7V. Design the circuit so that a current of 2mA flows through collector and a voltage of +5V appears at the collector.

BJT equations

The voltage at the emitter was measured and found to be -0.7V. If beta=50, find IE, IB, IC and VC.

BJT equations

A given npn transistor has beta=100. Determine the region of operation if:

a) IB=50uA and IC=3mA

b) IB=50uA and VCE=5V

c) VBE=-2V and VCE=-1V

BJT equations

RB=200kΩ, RC=1kΩ, VCC=15V, beta=100. Solve for IC and VCE

BJT equations

RB=200kΩ, RC=1kΩ, VCC=15V, beta=100. Solve for IC and VCE